1. Field of the Invention
The present invention is directed to a method for examining a body region in an examination subject, the body region executing a periodic motion within the examination subject, wherein data are acquired from the examination subject while irradiating the examination subject with penetrating radiation.
2. Description of the Prior Art
Examination of the beating heart with a CT apparatus such as, for example, for determining the degree of calcification (xe2x80x9ccalcium scoringxe2x80x9d) or for searching for stenosis in the coronary vessels, requires special measures for acquiring images that are low in motion artifacts. The ECG-triggered, sequential exposure technique is currently employed in various models of CT systems such as CT apparatuses of the 3rd generation (x-ray source and detector system rotate in common around a system axis) or electron beam CT apparatus as well (EBT=electron beam tomography). The R-wave of the ECG signal of the patient is used to implement a scan at a specific z-position, i.e. at a specific position in the direction of the system axis, in a defined phase of the cardiac cycle. The scan is thereby triggered with a selectable delay after a registered R-wave. The delay, and thus the starting time of the scan following the R-wave are usually to be determined from the current R-R interval duration TRR (for example, as a percentage). Since this must be prospectively estimated from the duration of the preceding cardiac cycles, the method is susceptible to arrhythmia of the heartbeat.
ECG-gated spiral technique is becoming popular for examinations of the heart, particularly with the introduction of multi-slice CT apparatuses, i.e. CT apparatuses having a radiation detector composed of a number of detector lines of individual detectors. With the assistance of the ECG signal, data intervals are retrospectively selected in order to be able to continuously image the heart volume in a defined phase, the selection being made from a scan registered given continuous rotation of x-ray source and detector system around the system axis with simultaneous, continuous displacement of patient and x-ray source relative to one another in the direction of the system axis. This is known as the multi-slice spiral technique and has important advantages over the ECG-triggered sequential exposure technique. The continuous data acquisition allows the reconstruction of overlapping tomograms, and thus a considerable improvement in the 3D image quality. The noticeable increase of the scan speed enables examinations with thinner slice thicknesses during a breath-holding phase, and thus a further improvement of the longitudinal resolution, i.e. the resolution in the direction of the system axis. The position of the data intervals in the cardiac cycle, i.e. of those time intervals during which data that can be interpreted for the reconstruction of tomograms are acquired, is not based on a prospective estimate but on the correctly measured R-R interval lengths TRR, so that an enhanced stability of the method given heart rate arrhythmia is achieved. Moreover, the heart volume in an arbitrary phase of the cardiac cycle can be calculated from the same dataset, as basis for functional examinations.
For most applications of retrospectively ECG-gated multi-layer spiral technique, the heart volume is to be reconstructed only in one heart phase, optimally free of motion artifacts. To this end, the gating parameters (delay of the R-wave) are selected such that all images are calculated in the low-motion diastole. This, however, means that the x-ray dose applied outside the correspondingly selected data intervals is not employed for the image reconstruction. Standard methods use, for example, data from a time interval of 350 msec in a cardiac cycle. For a heart cycle duration of 1 sec (pulse 60), this means 65% unused x-ray dose.
An object of the present invention is to provide a method of the type initially described wherein a body region of an examination subject executing a periodic motion can be registered with reduced dose.
The above object is achieved in accordance with the principles of the present invention in a method for examining a body region which executes a periodic motion in an examination subject, including the steps of irradiating the body region with penetrating radiation and detecting radiation exiting from the examination subject with a detector system which produces output data dependent on the radiation incident thereon, modulating the intensity of the radiation from the x-ray source between a reference and a value that is reduced compared to the reference value, substantially synchronously with the periodic motion, so that the reference value is present during a phase of the periodic motion, and calculating an image of the body region executing the periodic motion using output data from the detector system which were obtained only while radiation having an intensity at the reference value was emitted from the radiation source.
In the inventive method, thus, an ECG-controlled modulation of the x-ray dose is undertaken, for example by controlling the x-ray current, and desired rated dose value thus is reached only in the phase of interest in the cardiac cycle. The inventive method can be technically realized in a simple way and is distinguished by high flexibility and stability given patients with arrhythmia. The modulation of the x-ray dose by an adaptation of the tube current is preferred because of its simplicity. Nonetheless, other procedures such as, forexample, mechanical beam pre-filtering, are conceivable.
A location-dependent modulation of the x-ray dose is currently employed for examinations of, for example, the shoulder and the pelvis but is also employed at the thorax. Here, the x-ray dose is regulated dependent on the anticipated attenuation in the subject for the tube position under consideration in order to obtain a uniform image impression with a reduced overall dose (see U.S. Pat. No. 5,822,393). The inventive ECG-controlled modulation of the x-ray dose can be coupled with this established method, and thus a simultaneously location-dependent and time-dependent modulation of the x-ray dose can be undertaken.
Retrospectively ECG-gated multi-slice spiral technique allows the continuous imaging of the heart volume with thin, overlapping slices in arbitrary phase. A gap-free volume coverage in z-direction within a region dependent on the feed rate, and thus on the pitch can be achieved with suitable reconstruction and weighting methods (for example, projection-dependent weighting among the data of the individual detector lines). The feed rate is selected dependent on the period duration of the heart cycles, i.e. on the cardiac frequency, and taking the detector width into consideration, such that a sub-revolution or full-revolution dataset can be registered in every cardiac cycle and such that the regions covered by successive datasets overlap in z-direction orxe2x80x94in the limit casexe2x80x94abut gap-free. The entire heart volume in the z-direction can then be covered gap-free with tomograms or be displayed in a 3D image. For reconstruction of a 3D image in a specific phase (for example, the diastole) of the cardiac cycle, only data acquired during this phase are utilized for the reconstruction. The selection of the data ensues with a specific time spacing from the most recent R-wave that can, for example retrospectively, be determined as a fixed fraction of the known R-R interval length TRR. The x-ray dose by means of the tube current is regulated with the ECG signal such that a specific reference value of the x-ray dose exists during the data registration for this phase, but is otherwise regulated down to a considerably lower value (for example, ⅕ of the reference value).